Recent evidence suggests that a class of bone marrow derived mononuclear cells termed dendritic cells (DC), distinct from lymphocytes and conventional macrophages (MP), may be the critical "passenger leukocyte" of transplantation. DC have been found in the T-cell zones of the lymphoid organs and in the epidermis where they presumably function in antigen presentation. Similar cells have been described in the connective tissues of all the major organs including heart, kidney, liver, pancreas and thyroid. With the renewed interest in organ transplantation as a result of the development of new immunosuppressive agents it becomes desirable to understand how the DC might participate at the graft site. Little is known of the effect of cytotoxic or immunosuppresive drugs on these cells. Therefore we propose to examine the role of the DC in the allograft rejection process using a murine cardiac transplantation model. Free grafts of newborn mouse heart are placed in the pinna of the adult mouse ear. Mice with isogenic and allogeneic grafts will be harvested at days 4, 8, 14 and 28 days post-transplantation and sections will be cut and examined for DC specific cell surface and cytoplasmic markers (Ia antigen and S-100 protein) by immunocytochemical techniques. Conventional macrophages will also be identified by characteristic enzyme histochemical and lectin-binding patterns (acid phosphatase and peanut agglutinin). Use of T-cell markers (Thy 1 and Ly 1 and 2) will demonstrate any interaction between DC and the different functional classes of T-lymphocytes. Some of these markers are well preserved through conventional paraffin processing and will be used to identify DC and MP in a retrospective study of sections from an early murine heart allograft experiment in this laboratory. The donor or host origin of DCs participating in graft rejection phenomena will be determined by histopathologic evaluation of engraftments with Iak + and Iak - murine strains. Electron microscopic examination and immunocytochemical analysis of the transplant preparations will be performed to determine the ultrastructural characteristics of the mononuclear infiltrates. DC, MP and T-cell markers will be employed to analyse the effect of multiple and single reagent immunosuppressive therapy with antithymocyte serum and cyclosporin A on these cell types during prolonged cardiac allograft survival. Methods to deplete donor tissue of DCs and the effects of these pretreatment techniques on graft survival will be explored. Finally, isolation, purification and infusion of cardiac DC will demonstrate whether these cells alone can accelerate the rejection of a DC depleted cardiac allograft.